6.1 Introduction

So far we have only looked at functions that take a single input. In order to have a richer set of descriptors for a variety of processes, it is sometimes useful to recognize that relations may depend on more than a single input. For example, a model for a person's salary is likely to have at least two inputs, education and experience.

Likewise, the amount of money realized by an investment depends on both the interest rate and the length of time that the investment is held.

This leads to the idea of multivariate functions that depend on more than a single input.

A function of two variables, is a rule that takes two inputs and applies a rule to them to produce an output.